These bacteria differ from most other bacteria in that they can live and multiply only inside the cells of another organism (host) and cannot survive on their own in the environment.

For many species of these bacteria, small animals (such as rats and mice) are the usual host. Cattle, sheep, or goats are the hosts for Coxiella burnetii, which causes Q fever. Humans are the usual host for Rickettsia prowazekii, which causes epidemic typhus. These animals and humans—the hosts—are called the reservoir of infection. Host animals may or may not be ill from the infection. Rickettsiae and rickettsia-like bacteria are usually spread to people through the bites of ticks, mites, fleas, or lice that previously fed on an infected animal. Ticks, mites, fleas, and lice are called vectors because they convey (transmit) organisms that cause disease. Q fever, caused by Coxiella burnetii, can be spread through the air or in contaminated food and water and do not require a vector. Each species of rickettsiae and rickettsia-like bacteria has its own hosts and usually vectors.

Some of these bacteria (and the diseases they cause) occur worldwide. Others occur only in certain geographic regions.

Some of these bacteria infect the cells lining small blood vessels, causing the blood vessels to become inflamed or blocked or to bleed into the surrounding tissue. Other bacteria (Ehrlichia and Anaplasma) enter white blood cells.

Where damage occurs and how the body responds determine which symptoms develop.

Symptoms

Different rickettsial infections tend to cause similar symptoms:

Fever

Severe headache

A characteristic rash

A general feeling of illness (malaise)

A sore covered by a black scab (eschar) may form at the site of the bite. Because the rash often does not appear for several days, early rickettsial infection is often mistaken for a common viral infection, such as influenza. People may have swollen lymph nodes.

As the infection progresses, people typically experience confusion and severe weakness—often with cough, difficulty breathing, and sometimes vomiting.

When the infection is advanced, gangrene may develop, the liver or spleen may enlarge, the kidneys may malfunction, and blood pressure may fall dangerously low (causing shock). Death can result.

Diagnosis

A doctor's evaluation

Blood tests and biopsy of the rash

Because rickettsiae and rickettsia-like bacteria are transmitted by ticks, mites, fleas, and lice, doctors ask people

Whether they have been bitten by a tick or another vector

Whether they have traveled to an area where these infections are common

Being bitten is a helpful clue—particularly in geographic areas where rickettsial or a related infection is common. However, many people do not recall such a bite.

If doctors suspect Q fever, they may also ask whether people were at or near a farm (because cattle, sheep, and goats are the host for the bacteria that cause this infection).

Symptoms also help doctors diagnose these infections. Doctors ask people

How long it took for the rash to appear after they were bitten (if known)

Whether they have other symptoms.

A physical examination is done to determine which parts of the body are affected and what the rash looks like. Doctors also look for an eschar that people may not have noticed and for swollen lymph nodes.

Testing

Testing is usually needed to confirm the diagnosis. Often, doctors cannot confirm an infection with rickettsiae or rickettsia-like bacteria quickly because these bacteria cannot be identified using commonly available laboratory tests. Special blood tests for these bacteria are not routinely available and take so long to process that people usually need to be treated before test results are available. Doctors base their decision to treat on the person's symptoms and the likelihood of possible exposure.

Useful tests include

Blood tests that detect antibodies to rickettsiae or rickettsia-like bacteria

If people have a rash, removal of a small sample of affected skin for testing (biopsy)

Doctors use two techniques to make the bacteria easier to detect and identify:

In immunofluorescence assays, foreign substances produced by the bacteria (antigens) are labeled with a fluorescent dye.

The polymerase chain reaction (PCR) technique is used to increase the amount of the bacteria's DNA, so that the bacteria can be detected more rapidly.

Treatment

Antibiotics

Rickettsial infections respond promptly to early treatment with the antibiotics doxycycline (preferred) or chloramphenicol. These antibiotics are given by mouth unless people are very sick. In such cases, antibiotics are given intravenously. Most people noticeably improve in 1 or 2 days, and fever usually disappears in 2 to 3 days. People take the antibiotic for a minimum of 1 week—longer if the fever persists. When treatment begins late, improvement is slower and the fever lasts longer. If the infection is untreated or if treatment is begun too late, people may die, especially if they have epidemic typhus, scrub typhus, or Rocky Mountain spotted fever.

Ciprofloxacin and other similar antibiotics may be used to treat Mediterranean spotted fever but are usually not used to treat other rickettsial or related infections.

Rickettsia prowazekii, transmitted by lice or by unknown methods when the hosts are flying squirrels

People and flying squirrels

Throughout the world (uncommon in the United States, but occasionally occurs in people who have had contact with flying squirrels)

About 7 to 14 days after the bacteria enter the body, symptoms begin suddenly, with fever, headache, and extreme fatigue (prostration). A rash appears on the 4th to 6th day. Untreated, the infection may be fatal, especially in people older than 50.

Murine typhus

Rickettsia typhi or Rickettsia felis, transmitted by fleas

Cats, rodents, and opossums

Throughout the world

About 8 to 16 days after the bacteria enter the body, symptoms begin and are similar to those of epidemic typhus but are less severe.

About 6 to 21 days after the bacteria enter the body, symptoms begin suddenly, with fever, chills, headache, and swollen lymph nodes. A black scab may develop at the site of the chigger bite. A rash appears on the 5th to 8th day.

About 12 days after a tick bite, symptoms usually begin. They include fever, chills, muscle aches, weakness, nausea and/or vomiting, cough, headache, and a general feeling of illness. A rash may develop on the torso, arms, and legs

Granulocytic anaplasmosis

Anaplasma phagocytophilum, transmitted by ticks

Mainly mice and other small rodents

In the United States, the Northeast, mid-Atlantic, upper Midwest, and West Coast

Europe

About 12 days after a tick bite, symptoms usually begin. They include fever, chills, muscle aches, weakness, nausea and/or vomiting, cough, headache, and malaise.

About 9 to 28 days after bacteria enter the body, symptoms begin suddenly. They include fever, severe headache, chills, extreme weakness, muscle aches, loss of appetite, sweating, an unproductive cough, chest pain, and shortness of breath (caused by pneumonia), but no rash.

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